1. Field of the Invention
The present invention relates to a light emitting optical device capable of emitting light in the green-blue to ultraviolet spectrum, comprising a gallium nitride active layer on a silicon carbide substrate, and a method for making the same.
2. Description of the Related Art
Electroluminescent semiconductor devices such as lasers and light-emitting diodes (LEDs) that emit light in the green-blue to ultraviolet region of the electromagnetic spectrum are of great interest, but have not yet reached the levels of performance that are presently available in red and yellow light emitters, as measured by quantum efficiency and luminous intensity. The major reason for the deficiency of the blue light-emitting devices is the much less well developed state of the semiconductor materials that are suitable for blue light emission.
The semiconductor materials principally used for blue light emission are the II-VI compounds, silicon carbide, and the III-V nitrides.
The II-VI compounds such as ZnSe, ZnS, CdS, and their alloys are direct band gap materials and thus have high optical efficiency. However, while LEDs and lasers have been demonstrated in II-VI compounds, the lack of stability and the short lifetimes of devices made from them are serious concerns. Recently, it has been demonstrated that the short lifetimes in II-VI devices result from the rapid propagation of defects throughout their active regions, which act as nonradiative recombination sites. These results are indicative of low temperature and room temperature characteristics. Operation at higher temperatures or at high powers will increase the degradation rate and exacerbate the problem. Additional problems with the II-VI materials include the necessity of using quaternary layers to achieve blue emission in relatively lattice-matched heterostructures. The simplest and most well developed materials are ZnSe, ZnS, CdS, and their alloys which produce emission in the blue-green region, about 500 nm. Other II-VI compounds could be used to produce emission in the blue and ultraviolet, for example alloys of ZnS and either ZnSe, CdS, CdSe or MnSe. However, these materials have not been well investigated and have other major problems including the lack of a simple lattice-matched heterostructure system and the lack of a suitable substrate.
Blue LEDs formed in silicon carbide epilayers on silicon carbide substrates have been reported (U.S. Pat. No. 4,918,497). However, silicon carbide is an indirect band gap material, and therefore radiative recombination is inefficient, and consequently these SiC-based light emitting diodes (LEDs) have poor optical efficiency. A commercially available 6H-polytype SiC LED with a peak light emission at a wavelength of 470 nm has an external quantum efficiency of 0.02% and performance of 0.04 lumens/watt (Cree Research, Durham, N.C.). This performance is quite low compared to the best LEDs emitting in the red (AlGaAs, 16% quantum efficiency and 8 lumens/watt) and yellow-green (AlInGaP, 1% quantum efficiency and 6 lumens/watt), where quantum efficiency is defined as the number of photons emitted per electron supplied .times.100%, and luminous intensity is the luminous (visible) flux output of a light source measured in lumens divided by the electrical power input to the device. Lumens are calculated by multiplying the radiant flux output of a device (in watts) by the human eye's sensitivity as defined by the Commission Internationale de L'Eclairage (CIE), and so the luminous efficiency is related to the amount of light perceived by the human eye per power input.
The performance of SiC LEDs cannot be expected to match that of these III-V direct band gap materials. In addition, there is no convenient heterostructure system for SiC. A convenient heterostructure system is important because it increases device efficiency and permits selection of the emission wavelength.
The best choice for blue and UV light emitter applications appears to be the III-V nitrides, i.e. GaN, AlN, InN and their ternary and quaternary alloys such as AlGaN, InGaN, or AlInGaN. The III-V nitrides meet many of the requirements for making light emitting devices. These materials possess direct band gaps, a convenient, well lattice-matched heterostructure system, the ability to choose the output wavelength by varying the composition and structure, and good thermal stability. The III-V nitrides exhibit strong luminescence in the ultraviolet and blue.
These nitrides and their alloys are referred to herein as Ga*N materials. As used herein, the term Ga*N refers to binary (e.g. GaN), ternary (MGaN), and quaternary (MM'GaN) type gallium nitride compounds, including, by way of example, such compounds as AlN, InN, AlGaN, InGaN, InAlN, and AlInGaN, wherein M is a metal which is compatible with Ga and N in the composition MGaN and the composition MGaN is stable at standard temperature and pressure (25.degree. C. and one atmosphere pressure) conditions, and wherein M' is a compatible metal providing quaternary compounds of the formula M.sub.1-x-y M'.sub.y Ga.sub.x N, wherein x and y range from 0 to 1. It will be further understood that ternary and quaternary compounds may be referred to by general formula without subscripts, e.g., AlGaInN, wherein the stoichiometric coefficients (for aluminum, gallium, and indium, in this instance) have been deleted for general reference purposes, it being understood that such alloy compositions entail stoichiometry relative to the metal components which provides a stable composition at the aforementioned standard temperature and pressure conditions.
Recent improvements in growth and p-type doping have led to several demonstrations of high efficiency, GaN-based blue LEDs. GaN LEDs grown on (0001) oriented sapphire have exhibited an external quantum efficiency of 0.18%, almost 10 times that of SiC LEDs (S. Nakamura et al., Jpn. J. of Appl. Phys. 30 (1991) L1998). More recently, AlGaN/GaN heterostructure LEDs with outputs of 1000 mcd at 20 mA (Nikkei Electronics Dec. 20, 1993 (No. 597)) have been reported. By comparison, SiC LEDs emit only about 25 mcd at 20 mA. Additionally, stimulated emission by photopumping has been demonstrated in GaN (H. Amano et al., Jap. J. Appl. Phys. 29 (1990) L205); M. A. Khan et al., Appl. Phys. Lett. 58 (1991) 1515). Finally, the III-V nitrides possess many similarities to the III-V arsenides GaAs and AlAs, and so growth and fabrication techniques that have been well developed for the latter materials may be employed in fabricating nitride-based devices as well.
The band gap of GaN is 3.4 eV while that of AlN is 6.2 eV and InN is 2.09 eV. Thus a device with a GaN active layer would emit at about 365 nm for band to band recombination. Like GaAs/AlAs, GaN/AlN form a closely lattice-matched heterostructure system. The difference in the lattice constants of GaN and AlN is about 2.5%. While larger than the difference between the lattice constants of GaAs and AlAs, this fairly close match does permit the use of GaN/AlGaN alloys with mismatches of less than 0.5%. Such a convenient heterostructure system is important for light emitting devices because it increases device efficiency and permits selection of the emission wavelength. The wavelength can be modified in several ways. The first is through the use of quantum well heterostructures, in which the emission energy increases as the well width decreases, because of quantum size effects. The second is the use of AlGaN alloys in the active region, which also increases the emission energy relative to GaN. Of course, these two techniques could be combined, if desired. The emission energy can be reduced by the addition of In to the active region alloy.
A key problem with the present GaN light emitting devices is that they are primarily fabricated on (0001) oriented sapphire (Al.sub.2 O.sub.3) substrates. Use of sapphire has a number of problems. The first is that the lattice mismatch between GaN and sapphire is about 13.8%, which is quite large. This large lattice mismatch causes a high density of defects at the sapphire/GaN interface, in the range of 10.sup.8 to 10.sup.10 per cm.sup.2, and these defects propagate up into the device's active region during growth of the active layer. Defects pose a serious problem to the reliable operation of optical emitter devices such as lasers and LEDs. Often optical devices have dark line defects which multiply during operation. This phenomenon is an especially important problem for emitters grown on lattice mismatched substrates or containing layers that are mismatched. With continued operation, the density of these dark line defects increases until the light output is reduced to an unacceptable level and device failure occurs. Therefore, the device performance of GaN/sapphire light emitters is limited by crystal quality effects.
Another problem with GaN light emitting devices fabricated on sapphire substrates is that sapphire is insulating, which means that the standard, simple LED or laser structures, with one contact on top and the other on the bottom, cannot be used. Additional fabrication steps must be used to make both contacts on the tops of the devices. Additionally, specialized wirebonding and packaging must be employed to accommodate the non-standard arrangement of the two top contacts. An important side effect of having to place both contacts on top of the device when a sapphire substrate is used is that the die or chip area must be about twice as large as the standard structure to leave room for both contacts on the top surface. This effectively halves the number of devices which can be made on a given substrate area, resulting in increased cost per device.
It would therefore be advantageous to employ substrates other than sapphire for III-V nitride devices.
Silicon carbide has several advantages as a substrate material for III-V nitride based light emitting devices. The first is that it provides a much closer lattice match to the III-V nitrides than does sapphire (3.4% vs. 13.8% for sapphire and GaN), leading to devices with fewer defects and thus higher efficiencies and longer lifetimes. The second is that SiC can be made conductive, which permits the use of simple and conventional LED and laser structures, with one contact on top and the other on the bottom. Less processing is required thereby, the conventional packages and packaging tools can be used, and the device requires about one-half the area of an equivalent device made on a sapphire substrate. Finally, most polytypes of silicon carbide show high light transmittance throughout the visible light wavelengths and some show good transmittance even into the ultraviolet region.
GaN light emitting diodes that could be fabricated on bulk single crystal, single polytype SiC substrates, in particular (0001) oriented 6H-SiC or cubic 3C-SiC, have been proposed (U.S. Pat. No. 5,210,051). These combinations of materials could show significant advantages over the GaN/sapphire system. The hexagonal 2H-GaN/(0001)-oriented 6H-SiC system was selected because of the availability of the 6H-SiC substrate and the compatibility of the crystal structures. The cubic GaN/3C-SiC system was proposed likewise on the basis of crystal compatibility arguments, although bulk 3C-SiC substrates are not currently available and little is known of the preparation or properties of cubic GaN. The optimization of electrical and optical properties, in particular charge carrier mobilities and percent transmittance of light, have not been addressed, nor has the use of a heterostructure system, important because it increases device efficiency and permits selection of the emission wavelength. Methods to economically fabricate large quantities of GaN LEDs on SiC substrates have not been described. Device structures designed to increase the output of light emitted are needed. These issues are crucial to the economical production of high brightness, commercially viable blue light emitting devices.
It is therefore an object of the present invention to provide bright green-blue to ultraviolet light emitting devices with high optical efficiency, long lifetime, and a simple fabrication process which is compatible with current device fabrication, testing and packaging processes. The invention provides improved charge carrier mobilities and light transmittance. It is a further object of the invention to provide a method for making these bright blue light emitting devices.
It is a further object of this invention to provide a method for selection of the output wavelength of the light emitting device by control of the composition and structure of the active region of the device.
It is yet a further object of the invention to provide an alternative method for forming device definition during fabrication steps, to keep damage from the dicing operation, which may degrade the device characteristics, away from the active region, and thus to avoid the necessity of post-dicing etching step.
It is yet a further object of the invention to provide methods for reducing the misfit dislocation density at the interface between the substrate and the active layer.
It is yet a further object of the invention to provide a device structure that enhances the efficiency of light extraction from the light emitting device by means of Bragg mirrors that reflect light of the wavelength emitted by the device.
Other objects and aspects of the invention will be described in detail in the subsequent disclosure and claims.